Bromination of trans-Cinnamic Acid
[pic] Two chiral centers So: Four stereoisomers are possible (2n = 22 = 4) But: Only TWO are formed !
Theory: • Bromination of alkenes is stereospecific o Products contain 2 new chiral centers ▪ Products obtained from cis- alkenes are different from those obtained from trans-alkenes o Rationale: ▪ Formation of bromonium ion intermediate • NOT a “free” carbocation o A carbocation intermediate would lead to the formation of all possible stereoisomers. • Blocks one side of the molecule • SN2 attack by Br− from the opposite side leads to anti- addition overall ▪ Syn products are typically not formed (some exceptions do exist)
Experiment: Reaction: • Weigh 0.15g of trans- cinnamic acid into a reaction tube • Add 0.8 mL glacial acetic acid • Attach an air condenser and heat in a hot water bath (50oC), stirring occasionally, until the cinnamic acid dissolves. • Measure 1.0 mL of a 1.0 M solution of Br2 in acetic acid into a vial. • Use a Pasteur pipet to add this solution (to the cinnamic acid solution) through the top of the condenser. • Heat the reaction mixture in a hot water bath (50oC) until the red-brown color fades to light orange • Continue to heat for 15 min. • During this time: Plan Ahead! o Set up the filtration apparatus o Prepare an ice-water bath o Place a reaction tube containing deionized water in the ice-water bath to chill ▪ you will need it during filtration for washing/rinsing • If the mixture becomes colorless (or nearly so) during this period, add more Br2/acetic acid solution, dropwise, until the color just persists. • If the mixture has a distinct orange color at the end of the reaction period, add a drop or two of cyclohexene: it will turn light yellow.
Isolation: • Cool the reaction mixture in an ice-water bath for at least 10 minutes, scratching the sides of the tube to induce crystallization, if necessary. • Collect the product by vacuum filtration. • Wash with several portions of ice-cold water, until the acetic acid…